A recurring problem with all types of semi-permeable membrane systems used in the treatment of aqueous liquids is that microorganisms may cling to the fluid/membrane interface and multiply. If there is no control for these microorganisms they may grow quickly, fouling and clogging the membrane, especially when the liquid is not flowing.
Several workers in the field have recognized this problem. Beauman et al., in their 1983 U.S. Pat. No. 4,396,512 (assignee Everpure), provide a minor portion of silver bearing inert fibrous material, preferably cellulose, combined with a major portion of activated carbon to inhibit microorganism growth. The cellulose has individual fibers with sizes of about 10-1,000 micrometers with diameters of about 2-25 micrometers. The density is 0.3 grams per cc by dry tapped volume and the cellulose is impregnated with silver providing 1-5% of dry weight. In Beauman et al, the silver is adsorbed to the inert material and the resulting mixture is used as a packing material in a filter.
The 1976 U.S. Pat. No. 3,734,897 to Stoy et al., (Czechoslovak Academy) discloses the use of biostatic silver or copper with copolymers where the metal ions form a chelate complex bond with the polymeric composition. The polymeric material is used to form membranes. Here "biostatic" is defined as an action which inhibits microorganism growth. Stoy, et al, teach a complex, expensive method of manufacturing the silver or copper bearing membrane. The resulting membrane, since the biostatic metal is chemically bound, will not release all the metal and some of the metal cannot be used to inhibit microorganism growth. Thus more metal than necessary must be used.
Bettinger, in his 1979 U.S. Pat. No. 4,278,548 (assignee Dupont), adds iodide, hydrogen peroxide or derivatives directly to a liquid flow upstream of polyamide reverse osmosis membranes. These additions control the bacterial growth on the filter material and are useful for several months. Bettinger requires a separate injection capability which adds complexity and cost to the system.
An object of the present invention is the provision of a biostatic additive directly to and as part of semi-permeable membrane systems, for example, MF, UF, NF, RO, ED, EDI, PD and the like, which additive effectively inhibits the growth of microorganisms over a longer period compared to prior art systems. Such additives are easy to incorporate, economical to use and do not substantially interfere with the normal function of the membranes.
It is a further object of the present invention to provide a biostatic agent for semi-permeable membrane systems and the like which agent is effective in both flowing and non-flowing aqueous conditions (the latter conditions found when such membranes and assemblies of membranes are stored wet).
It is a further object of the present invention to use an economic quantity and form a a biostatic agent compared to prior art systems.